So just taken again the equation that we just found for the per-station
throughput. We said that a per-station throughput was
going to be the probability of transmitting times 1 minus the
probability of transmitting. And then we would do that multupliatin.
However, many stations we had minus 1, because the one that's transmitting we
don't have to include in this. So this is numstat minus 1 times.
So we see that This throughput is dependent on two things.
The first is the probability of transmitting, which we expected, and then
the seconds also the number of stations, okay, and so we don't really have control
over the number of stations that are under our access point because they will
come and go especially if it's a free access point, that's one of the
advantages of having a locked... [INAUDIBLE] besides the fact that people
can't steal your WiFi, then you don't have people actually disrupting your data
rate, as well, but what we can control is this probability of transmitting.
So, let's see what happens, and let's see if we can figure out what the throughput
will be as we change The probability of transmitting.
So in this graph right here, we're showing, for example five stations.
So we're saying that Numstat is equal to five in that equation.
So this really gets, this multiplication happens four times.
And then we're seeing what happens when we change the probability of a station
actually transmitting. And so, we can look at either the total
or the per-station throughput here it doesn't really make much of a difference
because it's just a multiplication factor as we said.
We just, to get the total throughput we just multiply this again by the number of
stations we have and that's how we translate from, from this graph up to the
top. And the higher one, total throughput will
always be greater than the per-station throughput.
Obviously, because we're just considering more stations.
But you see this trend here that first it's going to increase.
Right? And then it's going to hit some maximum
value before then it starts to taper off and decrease.
And this fits our intuition that we don't want it to be zero, because then the
throughput will be zero, and we also don't want it to be one because then
we'll always have collisions, because that's 100% transmission.
It's somewhere in between but it's skewed now towards the lower side.
And that maximum we hit we can see right here graphically, is when the probability
of transmission is at 20%. So we can say the best ProbTrans is 20%,
so then we might say from this graph, well okay then let's set probability of
transmission to be 20%. So then one in every five frames, on
average, every station will transmit. Notice how there are five stations.
So the next question is, well, are we done?
Are we satisfied with this 20% probability of transmission.
Clearly we can't do any better if we have five stations.
We see here it's, we're getting our maximum.
We don't have anything else to play with. There's no other knobs that we can turn
or anything to change any other parameters.
And, so, but it turns out that as we change the number of stations we're
going to see a little bit of a different story.
Okay? So, that what we just saw was for NumStat
which we're showing here equal to five. And now, this is a graph That's showing
what happens as NumStat varies. So it's just a bunch of superimposed
plots. The first one in the blue over here, this
straight line is for NumStat equal to one and that should make sense that if we had
one station, we would always want that station to transmit.
Because then we get 100% throughput, because we never have to worry about
interfering, so if you're the only station, go ahead.
You can get your maximum possible throughput and your really impressive
data rate, but once we have even two stations, that's when these things start
to taper off. Right, and that's when the throughput
starts to decrease. So if you notice a few trends about this
graph. The first thing is that the maximum that
we can achieve goes down. And it goes down pretty quickly as the
number of stations increases. So we have one station we can get 100%.
Two stations, the maximum we could get was 0.5.
Three stations, the maximum we can get is up over here and then with five stations,
it's down here. With ten stations, it's down here.
And so, as you can see, [INAUDIBLE] as soon as we add a second station in, we
already have had their total throughput that we can get.
So, and that's not even saying that pre-station throughput is half, that each
one gets half. It's the total throughput.
So when you add the two throughputs up, they're less than what one station can
get if the one station was there just by itself.
So that's a pretty unfortunate situation, clearly.
The second thing that you'll notice is the probability of transmitting.
The ideal, the optimal probability of transmitting, or the best one that we can
select goes down as the number of stations goes up.
Which should make sense because it's kind of along the lines that you have to wait
more. As there's more stations there because
there's more of a chance that someone else Has something to send.
Or someone else is going to send in that time slot.
So, prob-trans here, for two stations, the ideal is 50%.
So, half, and half, which should make sense.
For three stations it's 0.333. It's actually one third.
This is one half. For five station its actually 1 5th and
for 10 stations its 1 10th. You should know there is the trend there
and indeed that trend is the case that the [UNKNOWN] probability of transmitting
is simply going to be equal to one over the number of station [SOUND] For this
version of Aloha that we're looking at. So, if you could find the total number of
stations and you took one over that, and every device was following that, that
would be giving you the maximum total throughput that you could possibly get.
